A ball nut is a nut having a semi-circular helical groove on its inner diameter that fits over a shaft or ball screw having a mating semi-circular helical groove on its outer diameter. The load is transmitted by balls running in the grooves and returning in various manners through non-load carrying sections in the screw, nut or ancillary components.
Ball screw and nut assemblies are commonly utilized in extremely important control and actuation devices. One well-known application is the precise adjustment of flight control surfaces, as exemplified by U.S. Pat. No. 4,715,262 to Nelson et al., incorporated herein by reference. There are also many other precision control and machine applications. In particular, ball screw and nut assemblies are relied upon for ease of rotation, transforming rotation into a very precise lateral movement along the major axis of the screw.
While a wide variety of ball screw and nut designs are known, they share similar characteristics. Conventional ball screw nut assemblies commonly include a round leadscrew having a continuous helical groove or thread (with accompanying lands) along its length and a follower or nut with a mating continuous internal groove or thread that cooperates with the external groove of the leadscrew to form a course or race directing circulating bearing balls. The course is sized to contain a single-file row of a plurality of balls, which operate in rolling contact with both the leadscrew groove and the follower (or nut) groove as one is rotated relative to the other. The balls are displaced along the course as the leadscrew and follower rotate with respect to each other, facilitating easy and precise translation to lateral movement or motion.
The operation of this type of ball screw reduces frictional resistance, thereby saving power. The smooth relative rotation (as compared to other screw thread systems) and resulting smooth lateral movement facilitate high-speed operation. As a result, ball screw nut assemblies are often used in feed systems for cutting tools, or in other precision manufacturing applications.
In such ball screw and nut assemblies, the balls are caused to roll along the race or course by the relative rotation of the screw and nut. As a result, a structure for recirculating the balls is necessary. Conventional ball screw nut assemblies use a number of structures, techniques, etc., whereby the balls are recirculated, including the use of external and internal recirculation systems.
One aspect of a highly efficient operation of a ball screw nut assembly resides in the swift, unimpeded recirculation of the bearing balls. Examples of various types of external recirculating devices are found in the Nelson et al '262 patent, supra, and U.S. Pat. No. 6,109,415 to Morgan et al., both incorporated herein by reference. However, a drawback of this type of design having an external recirculation system of external tubes, channels, etc., is that the overall ball screw and nut configuration tends to be large and awkward. Consequently, various types of internal recirculating devices have been designed in order to address these difficulties.
One type of internal recirculating device is found in the embodiment shown in FIG. 5 of U.S. Pat. No. 5,832,776 to Kuo, incorporated herein by reference. However, it should be recognized that the recirculating or return path is merely a separate passage 51 formed in the nut or follower body 31. The passage runs parallel with the center axis of the main shaft of the leadscrew and it acts to channel or guide the balls from one end of the nut body to the other. This passage essentially acts in the same manner as the external recirculating tubes or channels depicted in the Morgan et al or Nelson et al patents, increasing the overall size of the device.
Additionally, a further difficulty encountered with this type of internal recirculation ball screw design is that the alignment of the return holes in the nut or follower may not form a sufficiently precise course or race for the easy recirculation of the balls. Sufficient precision must be maintained so that the balls transfer smoothly in both directions from the race or course to the follower or nut recirculation device. This makes the manufacturing process difficult and costly.
Moreover, if the balls are not constrained in precise alignment with each other, they may try to bypass each other and lock up, thereby causing complete failure of the ball screw and nut device. While this is an annoyance with machine tools, it can be disastrous in other applications of ball screw and nut assemblies, such as automobile steering, aeronautical flight control, etc.
This vulnerability becomes extremely pronounced when internal recirculating devices (also known as returns, crossbacks, switchovers, switchbacks, and flipbacks), such as that in the FIG. 2 of the Kuo patent, supra., are used. The return paths or switchbacks of Kuo return the balls to a previous position on an “upstream” groove to retrace the balls' forward path for each groove in the nut. This is effected by the use of return caps (32), which are squeezed into long and radial holes (34). An S-shaped sliding slot (321) is used to provide the return path for every two neighboring grooves. Because the S-shaped slots are relatively short, and must guide the balls through relatively sharp angles, the smooth flow of the balls may be impeded.
Moreover, manufacturing the arrangement set forth in FIG. 2 of Kuo requires a very complex process. The three-dimensional S-shape is difficult to align with any precision, except when substantially increased manufacturing times and complexities are involved. Even then, the use of the radial holes (34) for return caps (32) weakens the overall nut structure, as well as complicating the manufacturing process. Further, use of the radial holes (34) to insert return caps (32) in the Kuo device creates a danger that the ball bearings could escape through the holes if a cap were to fail. Besides the manufacturing complexity required for the Kuo device, it is also considered questionable for use in critical applications since the return caps could allow escape of the ball bearings and disastrous failure of the ball screw.
The difficulties produced by the device shown in FIG. 1 of the Kuo '776 patent have been addressed in a number of ways. One common technique is to eliminate the radial holes (34) and return caps (32) by substituting an insert into the threads along the length (major axis) of the nut or follower. Currently, separate inserts providing multiple S-shaped return paths are used in some ball screw and nut designs. FIG. 1 depicts a typical representation of a conventional design in which a continuous pattern of grooves (cuts 1 and 11) is ground or cut to form the interior threads of the nut body or follower (2). A portion of the threaded interior of the nut body (2) is cut out, i.e. slot (21), and a mating insert (3) is placed therein.
More particularly, in the conventional design shown in FIG. 1, insert (3) is fitted into nut body (2) by virtue of cutting out a section of the interior of nut (2), including portion of grooves (11) and (1). The rectangular cut out (21) must be sized and positioned so that insert (3) can be inserted in a manner that allows return grooves (31) to align with nut grooves (11) in the manner depicted in FIG. 1. The radial holes (34) and return caps (32) of the Kuo device are eliminated, and the manufacturing process is somewhat simplified.
It should be noted that FIG. 1 is merely a representation of a number of aspects of a conventional design, and is not an exact view of the ball screw nut assembly as it is arranged. In particular, the grooves of the screw are not shown at all. Rather, the view of FIG. 1 is of a nut that has been sectionalized to show the interior of the nut body. Superimposed upon this view is an additional view of insert (3), as it would appear for the inner face of the insert (3) that interfaces with the nut threads (11).
The recirculating grooves (31) of insert (3) reroute the balls (not shown) to align with an “upstream” thread (11) arranged ahead or “upstream” of the insert (3). The ball then restarts its forward advance along the race until it reaches insert (3) once again. In this conventional art arrangement, for selected grooves (11) of nut (2), there is a corresponding external thread groove (not shown) in a mating screw, and a recirculating or crossback groove (31) so that for each rotation of screw (1) with respect to nut (2) the bearing balls are recirculated.
While the aforementioned insert design of FIG. 1 avoids some of the difficulties described with respect to other conventional ball screws, there are still problems to this design. For example, in order to accommodate a practical thickness for insert (3), slot (21) must be cut very deeply through the interior threads (11) into the wall of nut (2). This substantially weakens the nut, as well as constitutes additional manufacturing effort. With the placement of insert (3) into nut (2), additional inaccuracies are introduced to the structure since some deformation of the insert or nut often occurs. This is especially critical due to the limited practical width of insert (3), which requires that the switchover path (31) encompass relatively sharp angles through which the balls must pass. Deformation of the insert could warp the flipovers or crossbacks (31) to the point that the movement of the balls is substantially impeded or even stopped. If the slot (21) is not precisely cut, or the insert (3) is not precisely sized, forcing the insert into the slot could also cause distortion of the insert, thereby degrading the flipover or crossback route (31).
Moreover, because the slot (21) that holds insert (3) must be deeply cut into the nut body to accommodate crossback grooves (31), the nut (2) is substantially weakened. Heat-treating of the nut to provide long-life ball grooves becomes problematical once the cut is made. Further, heat-treating may be impossible once the insert (3) has been inserted into slot (21). The result of all of these factors is a significantly weakened nut body, making its use in such critical systems as automobile steering problematical.
Additionally, several ball screw and nut assemblies have been developed directed to the use of intermediate rings, carriers, inert flexible tapes or strips, etc., to assist in guidance or the prevention of loss or the recirculating ball bearings. See, for example, U.S. Pat. No. 4,612,817. Similarly, multiple component ball screw and nut assemblies have been produced which utilize two or more pieces to produce the nut or screw components. See, for example, U.S. Pat. Nos. 3,393,575 and 3,393,576. However, the inclusion of these additional elements produces numerous manufacturing and assembly difficulties, all at increased cost.
Accordingly, there is a substantial need for improved ball screw and nut assemblies that overcome the aforementioned drawbacks of the conventional technology. Such an improved ball screw and nut assembly would be easier to manufacture than conventional models and be highly reliable, especially in regard to preventing escape of the bearing balls from the device.